Characterization of Carbon Nanomaterials Dispersions: Can Metal Decoration of MWCNTs Improve Their Physicochemical Properties?

A suitable dispersion of carbon materials (e.g., carbon nanotubes (CNTs)) in an appropriate dispersant media, is a prerequisite for many technological applications (e.g., additive purposes, functionalization, mechanical reinforced materials for electrolytes and electrodes for energy storage applications, etc.). Deep eutectic solvents (DES) have been considered as a promising “green” alternative, providing a versatile replacement to volatile organic solvents due to their unique physical-chemical properties, being recognized as low-volatility fluids with great dispersant ability. The present work aims to contribute to appraise the effect of the presence of MWCNTs and Ag-functionalized MWCNTs on the physicochemical properties (viscosity, density, conductivity, surface tension and refractive index) of glyceline (choline chloride and glycerol, 1:2), a Type III DES. To benefit from possible synergetic effects, AgMWCNTs were prepared through pulse reverse electrodeposition of Ag nanoparticles into MWCNTs. Pristine MWCNTs were used as reference material and water as reference dispersant media for comparison purposes. The effect of temperature (20 to 60 °C) and concentration on the physicochemical properties of the carbon dispersions (0.2–1.0 mg cm−3) were assessed. In all assessed physicochemical properties, AgMWCNTs outperformed pristine MWCNTs dispersions. A paradoxical effect was found in the viscosity trend in glyceline media, in which a marked decrease in the viscosity was found for the MWCNTs and AgMWCNTs materials at lower temperatures. All physicochemical parameters were statistically analyzed using a two-way analysis of variance (ANOVA), at a 5% level of significance.


Introduction
Carbon nanotubes (CNTs) were first reported by Iijima [1], receiving much interest due to their properties such as high aspect ratio, lightweight, high tensile strength, excellent electrical conductivity and stability [2][3][4]. The outer layer of the CNTs can be used as an anchor for the deposition of metallic nanoparticles (NPs) [3]. NPs incorporation is of great interest because they can form hybrid nanostructures that exhibit enhanced [5] or novel properties, due to size, large surface area and quantum dimension, which can enhance their physical and chemical properties [6]. Several metals (e.g., Ag, Au, and Pt) have been explored for the decoration of CNTs [7][8][9][10]. However, Ag nanoparticles (Ag-NPs) have Before the physicochemical studies, glyceline water content (wt.%) was determined using a Karl Fischer titrator (831 KF Coulometer, Methrom (Herisau, Switzerland)). The sample solution was manually mixed to achieve the maximum homogenization before titrating. A 1 cm 3 of a sample (~1.15 g) was added to the dry methanol solvent (HY-DRANAL™, max 0.01 wt.% water, Riedel−de−Haën (Honeywell Specialty Chemicals Seelze GmbH Charlotte, USA)) and titrated with HYDRANAL™ Composite 5 Reagent (4.5-5.5 mg mL −1 water equivalent, Riedel−de−Haën (Honeywell Specialty Chemicals Seelze GmbH Charlotte, USA)) for moisture determination. Measurements were performed in triplicate and the water content in glyceline was found to be 6.3 ± 0.5 wt.%.

Electrochemical Synthesis of Ag Nanoparticles and Decoration of MWCNTs
The electrochemical synthesis of Ag-NPs on the surface of MWCNTs was previously described by Brandão et al. [24]. Briefly, AgMWCNTs/DES were synthesized by a pulse reverse current mode (pe 86CB 3HE, plating electronic GmbH), at room temperature, using a two-electrode cell configuration with 50 mL of glyceline. Both electrodes were composed of Ag wires with an exposed area of approximately 5 cm 2 . Before each synthesis, the Ag wires were hand polished and washed with distilled water, and dried.
Different applied anodic and cathodic currents, and different on (t ON ) and off-times (t OFF ) were used to prepare the Samples A to E, details of which are presented in Table  S1, in Supplementary Materials. All electrodepositions were performed under magnetic stirring or ultrasonication. To recover the AgMWCNT, the electrolyte was centrifuged at 4000 rpm for 20 min, with intermediary washing with ethanol and hot water to remove the eutectic mixture. After AgMWCNT separation, the material was dried overnight at 60 • C and used without any further treatment. The main characteristics (amount, size, and specific capacitance) of the materials A to E are listed in Table 1.

Zeta Potential Analysis
The zeta potential (ZP) of particles was determined by electrophoretic mobility using the Helmholtz-Smoluchowski equation [49] through Zetasizer Nano ZS (Malvern Instruments, Malvern, UK). The values reported are the mean ± standard deviation of three different measurements recorded for each sample.
ZP was not calculated for the carbon-glyceline dispersions, due to the incapability of the equipment to measure these samples.

Physicochemical Characterization 2.4.1. Density and Dynamic Viscosity
Dynamic viscosities (η) and densities (ρ) of MWCNTs and AgMWCNTs dispersed in glyceline, and water were measured using the automated Anton Paar DMA™ 4500 M Nanomaterials 2022, 12, 99 4 of 23 micro viscometer (Anton Paar GmbH, Graz, Austria). The density measurement is based on electromagnetically induced oscillations of a U-shaped glass tube. The standard deviations associated with the density measurement were below 0.00005 g cm −3 . The viscosities were measured based on Hoeppler's falling ball principle. A 100 mm long capillary with a diameter of 3.0 mm (glyceline) or 1.6 mm (water) with a tilting angle from 24 • to 70 • . The standard deviation in dynamic viscosities data was below 0.5%.
All the physicochemical characterization measurements were performed in the temperature range between 20 • C and 60 • C, with 10 • C steps.

Refractive Index
Refractive index (RI) of both MWCNTs and AgMWCNTs dispersions in glyceline and water were determined using an Abbe Refractometer DR-A1-Plus (Atago Co, Ltd., Tokyo, Japan) with a resolution of ±0.00001 and uncertainty in the experimental measurements of ±0.0002. The apparatus was calibrated by measuring the refractive index of ultrapure water before the measurements, taking into consideration the values of RI vs. temperature of water already published [50].

Surface Tension
The measurement of the surface tension was performed in a homemade analyzer with temperature control, which is composed of a camera (Guppy F-036, Allied-vision, Stadtroda, Germany), and a sample holder. The measurement procedure was calibrated through the measurement of several drops of ultra-pure water at different temperatures. For every dispersion, 5 drops were formed to obtain an average and standard deviation. Once the drop is placed, an image is taken. The image is digitally processed and analyzed using the ImageJ software (W. Rasband, National Institutes of Health, Bethesda, MD, USA) with the Pendent Drop plug-in (Adrian Daerr, Paris, France) [51].

Ionic Conductivity
The ionic conductivity of the MWCNTs dispersions in glyceline and water was measured with Mettler Toledo Conductivity meter F30 (Columbus, OH, USA). An aqueous solution of KCl was used as a calibration standard. The temperature was stabilized with a thermostat for both calibration and experimental measurement.

Statistical Analysis
Statistical analysis (GraphPad Prism 9 software (Graphpad Holdings, San Diego, CA, USA) was employed to perform a two-way analysis of variance (ANOVA) of the experimental data at a 5% level of significance. Linear regression was performed to establish the correlation of every determined parameter with the evolution of temperature and concentration. All determinations were performed in triplicate and a relevant average value and standard deviation are reported.

Morphological Characterization
The morphological characterization of MWCNTs and Ag-MWCNTs composite samples prepared in this work was performed through SEM analysis, and the images are presented in Figure 1.
The SEM analysis for Samples A to E are displayed in Figure 1, showing the morphology of the Ag-MWCNTs composites with different electrodeposition parameters as detailed in Table 1. SEM analysis of AgMWCNTs (A-E) shows that Ag-NPs are located on the edges and surface of the MWCNTs, and presents some agglomerates of Ag-NPs (more evident in Samples D and E). The average diameter of the Ag-NPs is presented in Table 1. Sample E presents the higher amount of Ag, bigger size of Ag-NPs, and higher specific capacitance as previously published by Brandão et al. [24].

Morphological Characterization
The morphological characterization of MWCNTs and Ag-MWCNTs composite samples prepared in this work was performed through SEM analysis, and the images are presented in Figure 1. The SEM analysis for Samples A to E are displayed in Figure 1, showing the morphology of the Ag-MWCNTs composites with different electrodeposition parameters as detailed in Table 1. SEM analysis of AgMWCNTs (A-E) shows that Ag-NPs are located on the edges and surface of the MWCNTs, and presents some agglomerates of Ag-NPs (more evident in Samples D and E). The average diameter of the Ag-NPs is presented in Table 1. Sample E presents the higher amount of Ag, bigger size of Ag-NPs, and higher specific capacitance as previously published by Brandão et al. [24].

Zeta Potential Analysis
The ZP analysis was performed for MWCNTs and AgMWCNTs samples dispersed in water, and the results are presented in Figure 2. The ZP obtained for the MWCNTs and the different AgMWCNTs samples, using a 0.4 mg cm −3 dispersion (Figure 2a) shows that introducing the AgMWCNTs leads to the increase, towards more negative potentials, of ZP. This seems to be strictly associated to the presence of AgNPs, in particular with the increase of Ag NPs size. According to Sabri et al. [52], a high ZP value correlates to stable dispersion and the dispersed MWCNTs are well distributed. While the low ZP derives an attractive force that exceeds the repulsive force, the dispersion of the MWCNTs tends to aggregate. This is in line with the obtained results (Figure 2b,c), where an increase in the concentration of carbon materials in the water dispersions, leads to a decrease in the ZP values.

Zeta Potential Analysis
The ZP analysis was performed for MWCNTs and AgMWCNTs samples dispersed in water, and the results are presented in Figure 2. The ZP obtained for the MWCNTs and the different AgMWCNTs samples, using a 0.4 mg cm −3 dispersion (Figure 2a) shows that introducing the AgMWCNTs leads to the increase, towards more negative potentials, of ZP. This seems to be strictly associated to the presence of AgNPs, in particular with the increase of Ag NPs size. According to Sabri et al. [52], a high ZP value correlates to stable dispersion and the dispersed MWCNTs are well distributed. While the low ZP derives an attractive force that exceeds the repulsive force, the dispersion of the MWCNTs tends to aggregate. This is in line with the obtained results ( Figure 2b,c), where an increase in the concentration of carbon materials in the water dispersions, leads to a decrease in the ZP values.  consideration the overall results, the decoration of MWCNT with Ag nanoparticles significantly changed the zeta potential (from ≈−1 mV to −33 mV AgMWCNT, Sample E). This change in zeta potential for modified MWCNTs is in agreement with other published work [53] and is compatible with increased stability of AgMWCNTs dispersions in water, making it suitable for the characterization of the physicochemical properties.

Physicochemical Properties of MWCNTs and AgMWCNTs
The physicochemical properties of MWCNTs and AgMWCNTs dispersions in glyceline, and water were measured and will be thoroughly discussed in this section. A detailed comparison will be presented for AgMWCNTs Samples A through E since they cover different features of the prepared AgMWCNTs, namely the effect of mass transport on the synthesis step (comparison between Samples B and E); the effect of the amount of Ag and size of Ag-NPs (comparison between Samples C and D, B and E and B and C). The detailed information regarding the parameters for the electrodeposition process of AgMWCNTs can be found in the Supplementary Materials (Table S1).
The detailed information regarding the physicochemical parameters of MWCNTs and AgMWCNTs dispersions can be found in Supplementary Materials, and it will be referred throughout the text as necessary.

Density
Solvent densities (ultrapure water and glyceline) were measured to allow the validation of the experimental procedure and to build a set of reference points to help understand the changes observed.
The measured density values for MWCNTs dispersed in glyceline and water, as a function of temperature is presented in Figure 3a,b respectively. The temperature dependence in glyceline ( Figure 3a) is linear, however, in water (Figure 3b), it presents a slight curve. The linear decrease that can be seen in DES is due to the formation of larger intermolecular voids at higher temperatures, which increase the volume and decrease the density [54]. This effect was already described by several authors [54][55][56][57].
The density values for the different dispersions both in glyceline and water present similar results; however, in both cases, the introduction of the MWCNTs increases the density. The effect of MWCNTs is less pronounced in water, which, as expected, also presents a lower density compared to glyceline. Density values for pure glyceline found in the literature are listed in Table 2 and have a large range of values (namely the values found for 25 • C and 90 • C) and only a reduced number of works (e.g., the work of Crespo et. al. [27] and the present work) display the water content of the eutectic mixture. The value determined in the present work is in good agreement with the value reported by Crespo et. al. [27]. The density of ultrapure water is also in good agreement (less than 0.01% difference) with published values [58].
The decrease in density as a function of temperature is expected since an increase in temperature increases the DES molecules mobility and due to that, the thermal expansion of the DES volume increases [54]. The most important feature of the temperature effect in density is the fact that glyceline and their dispersions have a linear relationship with temperature.
The effect of Ag-NPs attached to the MWCNTs surface, dispersed in glyceline and water media is presented in Figure 3c   The density values for the different dispersions both in glyceline and water present similar results; however, in both cases, the introduction of the MWCNTs increases the density. The effect of MWCNTs is less pronounced in water, which, as expected, also presents a lower density compared to glyceline. Density values for pure glyceline found in the literature are listed in Table 2 and have a large range of values (namely the values found for 25 °C and 90 °C) and only a reduced number of works (e.g., the work of Crespo et. al. [27] and the present work) display the water content of the eutectic mixture. The value determined in the present work is in good agreement with the value reported by Crespo et. al. [27]. The density of ultrapure water is also in good agreement (less than 0.01% difference) with published values [58].   [27] 0.98320 [58] Replacing MWCNTs by AgMWCNTs does not introduce any significant effect on the density of water dispersions (less than 0.01%). In glyceline, a slightly larger decrease in density is observed for all the samples in the temperature range between 20 and 60 • C (around 0.0633 ± 0.0003%).

Dynamic Viscosity
Viscosity is an important property that must be addressed, especially for equipment design and fluid flow calculations. Viscosity values for pure glyceline, found in the literature are listed in Table 3. The dynamic viscosity of ultrapure water is also in good agreement (less than 0.05% difference) with published values [58], indicating the quality of the measured values for glyceline and carbon dispersions. The viscosity as a function of temperature is presented in Figure 4, for different concentrations of MWCNTs in glyceline and water (Figure 4a,b respectively). The viscosity of water is significantly lower than glyceline, and the decrease of viscosity with the increase in temperature is observed for both media, being more pronounced in glyceline (90% decrease from 20 • C to 60 • C).
The introduction of MWCNTs in the dispersing media displays a different behavior in glyceline and water. There is a decrease in viscosity in glyceline and an increase in water when adding MWCNTs, which is in agreement with data reported in some research papers [61,62]. That inversion observed in glyceline was previously reported by Alizadeh and Moraveji [63], for graphene nanoplatelets dispersed in an ionic liquid which showed a decrease in viscosity when compared to the original ionic liquid. This phenomenon might be explained by the self-lubricating nature of carbonous materials. Lower viscosity can result in reducing the pressure drop penalty in thermo-fluids systems and increasing the hydraulic and energy efficiency as a result [64].
The effect of Ag-NPs attached to the MWCNTs surface, dispersed in glyceline and water media is presented in Figure 4c (Samples B, D and E), 4d (Samples A and C). To facilitate data interpretation Figure 4c,d present the ratio of η Ag-M WCNTs /η MWCNTs as a function of temperature for samples at a fixed concentration (0.4 mg cm −3 ). The measured dynamic viscosity values of MWCNTs and AgMWCNTs dispersions (Samples A to E) as a function of temperature in the range 20 • C to 60 • C are presented in SI, in Figure S3f, Tables S6 and S7 for glyceline and Figure S4f, Tables S8 and S9 for water.
The introduction of AgMWCNTs on glyceline media leads to a small increase in viscosity when compared to pristine MWCNTs. AgMWCNTs dispersions in water display a systematic increase in viscosity (when compared to MWCNTs dispersions) of around 1.6%. When this study is carried out in glyceline, the AgMWCNTs samples containing larger amounts of silver (Samples B, D and E) display a decrease in viscosity (a 5.0% decrease) while dispersions prepared with Samples A and C show an increase in viscosity (a 6.7% increase). Samples D and E (which have a larger amount of Ag) display a larger and more consistent decrease in viscosity.
Studying the viscosity change with temperature allows to retrieve important information that can be retrieved by fitting an Arrhenius type Equation (1): where η 0 is a constant, E η is the energy for activation of the viscous flow, R is the gas constant (8.31 J K −1 mol −1 ) and T is the temperature (K) [65]. Figure 5 shows that the data for MWCNTs dispersion in glyceline and water obey Equation (1) well (R 2 > 0.989).
Nanomaterials 2022, 12, x FOR PEER REVIEW 9 of 24 of water is significantly lower than glyceline, and the decrease of viscosity with the increase in temperature is observed for both media, being more pronounced in glyceline (90% decrease from 20 °C to 60 °C). The introduction of MWCNTs in the dispersing media displays a different behavior in glyceline and water. There is a decrease in viscosity in glyceline and an increase in water when adding MWCNTs, which is in agreement with data reported in some research papers [61,62]. That inversion observed in glyceline was previously reported by Alizadeh and Moraveji [63], for graphene nanoplatelets dispersed in an ionic liquid which showed a decrease in viscosity when compared to the original ionic liquid. This phenomenon might be explained by the self-lubricating nature of carbonous materials. Lower viscosity can result in reducing the pressure drop penalty in thermo-fluids systems and increasing the hydraulic and energy efficiency as a result [64].
The effect of Ag-NPs attached to the MWCNTs surface, dispersed in glyceline and water media is presented in Figure 4c   Tables 4 and 5 list the values of ln (η 0 ) (interception), E η /R (slope), and the regression coefficient (R 2 ) the model for viscosity/temperature in glyceline and water, respectively. The minimum R 2 was 0.996, for the higher concentration of MWCNTs in glyceline and water.
The calculated values of E η are summarized in Tables S10 and S11, in Supplementary Materials, for glyceline and water, respectively. The values of E η diminish when the concentration of increases from 0.2 to 1.0 mg cm −3 . This effect is observed for all samples tested in this study. It is known that lower activation energy is associated with more mobile ions within the melt [66]. mation that can be retrieved by fitting an Arrhenius type Equation (1): where η0 is a constant, Eη is the energy for activation of the viscous flow, R is the gas constant (8.31 J K −1 mol −1 ) and T is the temperature (K) [65]. Figure 5 shows that the data for MWCNTs dispersion in glyceline and water obey Equation (1) well (R 2 > 0.989).  Table 4 and Table 5 list the values of ln (η0) (interception), Eη/R (slope), and the regression coefficient (R 2 ) the model for viscosity/temperature in glyceline and water, respectively. The minimum R 2 was 0.996, for the higher concentration of MWCNTs in glyceline and water.   The values of E η follow the same trend described for the MWCNTs dispersions, diminishing when the concentration increases from 0.2 to 1.0 mg cm −3 . Tables S12 and S13 list the values of ln (η 0 ) (interception), E η /R (slope), and the regression coefficient (R 2 ) the model for viscosity/temperature in glyceline and water, respectively, for AgMWCNTs dispersions.

Surface Tension
The intermolecular attractive forces in a liquid result in cohesive tension that diminishes the surface area of the liquid's interface with other phases in contact with the liquid, a phenomenon identified as surface tension, which is measured as the energy required to increase the surface area of a liquid by a unit of area. Surface tension values for pure glyceline and water, found in the literature are listed in Table 6, presenting similar results.
The difference between the experimental values presented for glyceline with the ones reported in the literature may be attributed to a possible difference in the water content of DES however this cannot be verified since other authors did not provide that information.  Figure 6 shows the effect of concentration and temperature of MWCNTs in glyceline ( Figure 6a) and water (Figure 6b). All measured surface tension values of MWCNTs and AgMWCNTs (Samples A to E) as a function of temperature are presented in Supplementary Materials, in Tables S14-S17, respectively for water and glyceline dispersions. Figure S5 presents the surface tension of glyceline/water mixtures with MWCNTs and Ag-MWCNTs as a function of carbon concentration (a) and temperature (b and c).   Figure 6a,b show that both tested solvents display a decrease in surface tension with increasing temperature. The same trend was reported by Abbott et al. [68], where the surface tension is strictly correlated with the viscosity of the system, which will be discussed later. The eight different MWCNTs dispersions (four in each solvent) display the same trend of the pure solvents (Figure 6a,b) showing an increase in surface tension with the increase in MWCNTs concentration. A similar effect was already reported in the literature  Figure 6a,b show that both tested solvents display a decrease in surface tension with increasing temperature. The same trend was reported by Abbott et al. [68], where the surface tension is strictly correlated with the viscosity of the system, which will be discussed later. The eight different MWCNTs dispersions (four in each solvent) display the same trend of the pure solvents (Figure 6a,b) showing an increase in surface tension with the increase in MWCNTs concentration. A similar effect was already reported in the literature [69] and is more pronounced in glyceline than in water and reflects the reduced surface activity of the MWCNTs dispersed in glyceline or water. Figure 6c (Samples B, D and E) and 6d (Samples A and C) present the ratio of γ Ag-M WCNTs /γ MWCNTs as a function of temperature for AgMWCNTs dispersions at a fixed concentration (0.4 mg cm −3 ). Dispersions of AgMWCNTs display a further increase in surface tension in water (between 2 and 6% increase) and in glyceline (between 1 and 7% increase).
Surface tension data, presented in Figure 6a,b, was analyzed using the "hole theory" developed by Abbott et al. [70,71]. DES "hole theory" was developed by Abbott based on the initial model developed by Fürth [72], which can be used to explain the mobility of ions/particles [70,71]. The "hole theory" model was also considered as a valid model by Bockris et al. [73] to describe high-temperature molten salts. According to this model [74], ionic materials contain empty spaces constituted from thermally generated fluctuations in local density. The vacancies are of random size and its position are in constant motion. It is assumed that an ion can only move through an ionic liquid if it is adjacent to a hole of equal or greater size. The average hole size, r, in a liquid is given by the Equation (2) [71]: where k is the Boltzmann constant, γ is the surface tension, and T is the temperature. The results obtained for glyceline and MWCNTs dispersion are listed in Table 7. In the studied dispersions, taking into consideration the studied parameters (concentration, temperature, and presence of MWCNTs in glyceline) it is possible to verify, that the void radius decreases with the increase in the concentration of MWCNTs. The results were further compared with pure glyceline. At 20 • C, the DES presents a void radius of 4.216 Å, followed by the MWCNTs dispersion with 4.160 Å, at 0.2 mg cm −3 . The obtained values of void radius for the dispersion with MWCNTs in glyceline are lower than the carbon-free liquid, indicating that the viscosity decrease is probably controlled by the lubricating properties of MWCNTs [63] rather than the hole size [71].
The values for the average hole size, r, for the AgMWCNTs in glyceline, are presented in Tables S18 and S19 in the Supplementary Materials, revealing that the void radius increases when AgMWCNTs dispersions are used. However, this increase in the void radius is not reflected in a general decrease in viscosity, when compared to MWCNTs dispersions. Samples A and C show an increase in viscosity (Figure 4d) and Samples B, D and E (Figure 4c) show a decrease in viscosity. The presence of AgNPs at MWCNTs surface altered the force balance introducing new surface interactions with the DES components that reduced the MWCNTs lubricating properties but potentiate the increase in the void size. It is the delicate balance between these two opposite effects that define the effect over the dispersion average viscosity.

Ionic Conductivity
Electrochemical applications are an important field where DESs can have a significant contribution to the industry, so there is a great demand for information concerning their electrical and electronic properties, including their ionic conductivity. Due to their relatively high viscosity, DESs have low conductivity. In general, the conductivity of DESs increases as temperature increases [59].
Ionic conductivity values for pure glyceline and water, found in the literature are listed in Table 8. The values for ultrapure water is also in good agreement (less than 1.5% difference) with published values [58]. The ionic conductivity presented by the Milli-Q ® water manufacturer is also in good agreement with the obtained result at 20 • C.
The difference between the experimental values presented for glyceline with those reported in the literature may be attributed to a possible difference in the water content of DES this information could not be found and the presence of water in the eutectic mixture can help increase the conductivity [75]. The conductivity as a function of temperature is represented in Figure 7, for different concentrations of MWCNTs in glyceline and water (Figure 7a,b respectively). For all the measurements, with increasing temperature, there is an increase in ionic conductivity.
The ionic conductivity of MWCNTs and AgMWCNTs mixtures (samples A to E) as a function of temperature in the range 20 • C to 60 • C are presented in Supplementary Materials, in Tables S20 and S21 and Tables S22 and S23, respectively for water and glyceline.
For all the MWCNTs measurements, in both glyceline and water, with increasing temperature, there is an increase in conductivity. Several studies have presented a considerable increase in conductivity of the fluids with the addition and dispersion of graphene NPs to them [77,78]. The increase of conductivity with the suspension of carbon materials, in this case, MWCNTs, can be attributed to the charge transfer enhancement which is significantly affected by EDL (electrical double layer) effect in solution [79]. For all the MWCNTs measurements, in both glyceline and water, with increasing temperature, there is an increase in conductivity. Several studies have presented a considerable increase in conductivity of the fluids with the addition and dispersion of graphene NPs to them [77,78]. The increase of conductivity with the suspension of carbon materials, in this case, MWCNTs, can be attributed to the charge transfer enhancement which is significantly affected by EDL (electrical double layer) effect in solution [79].
The effect of Ag-NPs attached to the MWCNTs surface, dispersed in glyceline and water media is presented in Figure 7c (Samples B, D, and E) and 7d (Samples A, and C). To facilitate data interpretation Figure 7c-d present the ratio of σAgMWCNTs/σMWCNTs as a function of temperature for samples at a fixed concentration (0.4 mg cm −3 ).
The effect of the presence of the Ag-NPs is very pronounced in this physicochemical property, especially in the glyceline media, showing a decrease in the effect of the Ag-NPs with an increase in temperature. As expected (Figures S6f and S7f, in supplementary materials), the Ag-NPs were able to contribute to a substantial increase of the ionic conductivity of dispersion media containing AgMWCNTs due to the high conductivity of silver (rAg = 6.30 × 10 5 S cm −1 ) [17].
The ionic conductivity changes, by the introduction of AgMWCNTs, presents an increase up to 20 times, compared to MWCNTs, especially in the sample with a higher amount and size of Ag-NPs (Sample E), followed by the samples with a lower amount of The effect of the presence of the Ag-NPs is very pronounced in this physicochemical property, especially in the glyceline media, showing a decrease in the effect of the Ag-NPs with an increase in temperature. As expected (Figures S6f and S7f, in Supplementary Materials), the Ag-NPs were able to contribute to a substantial increase of the ionic conductivity of dispersion media containing AgMWCNTs due to the high conductivity of silver (r Ag = 6.30 × 10 5 S cm −1 ) [17].
The ionic conductivity changes, by the introduction of AgMWCNTs, presents an increase up to 20 times, compared to MWCNTs, especially in the sample with a higher amount and size of Ag-NPs (Sample E), followed by the samples with a lower amount of Ag NPs, showing that both parameters are strictly correlated with the ionic conductivity. The increase in conductivity brought by the attachment of Ag-NPs to the MWCNTs surface was also presented earlier [17,[80][81][82][83].
The effect of temperature on the conductivity can be described according to Arrheniustype behavior (log σ vs. 1/T), as previously developed by Abbott et al. [68,76] and presented in Equation (3).
where σ is the conductivity at temperature T (K), σ 0 is the corresponding pre-exponential factor, E σ is the activation energy of ionic conductivity and R is the gas constant (8.31 J K −1 mol −1 ). Figure 8a,b show that the data fits Equation (3)  Ag NPs, showing that both parameters are strictly correlated with the ionic conductivity. The increase in conductivity brought by the attachment of Ag-NPs to the MWCNTs surface was also presented earlier [17,[80][81][82][83]. The effect of temperature on the conductivity can be described according to Arrhenius-type behavior (log σ vs. 1/T), as previously developed by Abbott et al. [68,76] and presented in Equation (3).
where σ is the conductivity at temperature T (K), σ0 is the corresponding pre-exponential factor, Eσ is the activation energy of ionic conductivity and R is the gas constant (8.31 J K −1 mol −1 ). Figure 8a,b show that the data fits Equation (3) accurately (R 2 > 0.998).  Tables 9 and 10 lists the values of ln (σ0) (interception), Eσ/R (slope), and the regression coefficient (R 2 ) the model for conductivity/temperature in glyceline and water, respectively. The minimum R 2 was 0.972, for the higher concentrations of MWCNTs in glyceline.   Tables 9 and 10 lists the values of ln (σ 0 ) (interception), E σ /R (slope), and the regression coefficient (R 2 ) the model for conductivity/temperature in glyceline and water, respectively. The minimum R 2 was 0.972, for the higher concentrations of MWCNTs in glyceline. Table 9. Conductivity-temperature model parameters for MWCNTs dispersed in glyceline. ln (σ 0 ) 6.9 ± 0.9 6.5 ± 0. The activation energy of ionic conductivity for the Samples A to E, in glyceline and water, are presented in Table S10 and S11, in Supplementary Materials. The E σ of water is higher, for all MWCNTs concentrations than glyceline. In both cases, there is a decrease in the E σ with an increase in MWCNTs concentration. Figures S6 and S7 show the fitting data for AgMWCNTs dispersion in glyceline and water, respectively which fits Equation (3) accurately (R 2 > 0.998). The presence of AgNPs in the MWCNTs significantly improves the Arrhenius-type behavior of glyceline ionic conductivity. Tables S24 and S25 list the values of ln (σ 0 ) (interception), E σ /R (slope), and the regression coefficient (R 2 ) the model for conductivity/temperature in glyceline and water, respectively, for AgMWCNTs dispersions.

MWCNTs Concentration/mg cm
Since the conductivity is controlled by the mobility of the charge carriers in ionic fluids, the plot of E σ vs. E η should be linear [68]. That is visible in Figure 9a, for glyceline, containing MWCNTs and AgMWCNTs. However, the same cannot be observed in water, presented in Figure 9b, since it is not an ionic fluid, as presented by Abbott et al. [68].
The activation energy of ionic conductivity for the Samples A to E, in glyceline and water, are presented in Table S10 and S11, in Supplementary Materials. The Eσ of water is higher, for all MWCNTs concentrations than glyceline. In both cases, there is a decrease in the Eσ with an increase in MWCNTs concentration. Figures S6 and S7 show the fitting data for AgMWCNTs dispersion in glyceline and water, respectively which fits Equation (3) accurately (R 2 > 0.998). The presence of AgNPs in the MWCNTs significantly improves the Arrhenius-type behavior of glyceline ionic conductivity. Tables S24 and S25 list the values of ln (σ0) (interception), Eσ/R (slope), and the regression coefficient (R 2 ) the model for conductivity/temperature in glyceline and water, respectively, for AgMWCNTs dispersions.
Since the conductivity is controlled by the mobility of the charge carriers in ionic fluids, the plot of Eσ vs. Eη should be linear [68]. That is visible in Figure 9a, for glyceline, containing MWCNTs and AgMWCNTs. However, the same cannot be observed in water, presented in Figure 9b, since it is not an ionic fluid, as presented by Abbott et al. [68].

Refractive Index
The experimental refractive index (RI) data in glyceline and water at room temperature, at different concentrations of MWCNTs, is presented in Figure 10a,b, respectively. Refractive index values for pure glyceline and water, found in the literature are listed in Table 11.

Refractive Index
The experimental refractive index (RI) data in glyceline and water at room temperature, at different concentrations of MWCNTs, is presented in Figure 10a,b, respectively. Refractive index values for pure glyceline and water, found in the literature are listed in Table 11.     Tables S26-S29, respectively for water and glyceline. Figure S8 presents the effect of concentration of MWCNTs and AgMWCNTs composites on refractive index in glyceline (a) and water (b) media, at 20 • C.
An increase in the concentration of MWCNTs is associated with an increase in the RI, in both glyceline and water. This can be rationalized by the fact that by increasing the concentration of MWCNTs, the availability of carbon materials in solution increases, raising the chances of light to hit a higher number of molecules, and thereby increasing the RI [81]. The effect of temperature was performed for MWCNTs in both glyceline and water, showing that increasing the temperature, the molecules speed increases in solution, causing the light to hit fewer molecules, and thereby reducing the RI [82,83].
The increasing trend of RI with increasing concentration of MWCNTs is also found in the literature [67]. The experimental RI data in glyceline and water at room temperature, at different concentrations of MWCNTs and AgMWCNTs, is presented in Figure S8. It can be observed that the increase in the concentration of MWCNTs and AgMWCNTs is associated with an increase in the RI, in both glyceline and water.
The effect of Ag-NPs attached to the MWCNTs surface, dispersed in glyceline and water media can be observed by analysis of Figure 10c (Samples B, D and E), 10d (Samples A and C). To facilitate data interpretation, Figure 10c,d present the ratio of RI AgM WCNTs / RI MWCNTs as a function of temperature for samples at a fixed concentration (0.4 mg cm −3 ).
Shahriari et al. [85] concluded that, with the growth of Ag-NPs size, there is an increase of the RI, due to the increase of diffusivity of the medium. That correlation can be observed for the AgMWCNTs samples in glyceline, especially for Samples B, C and D; however, it was expected that behavior for Sample E, which presents the bigger size of AgNPs, and that is not the case.

Statistical Analysis
The analysis of variance through two-way ANOVA, for viscosity, density, conductivity, and surface tension, as a function of the concentration of MWCNTs and AgMWCNTs (Samples A to E), are shown in Tables S30 and S31, in Supplementary Materials, for glyceline and water, respectively, with 5% level of significance. The statistical significance of the different studied physicochemical parameters depends on the p-value.
Starting with the analysis of the results from glyceline media, the concentration of both MWCNTs and AgMWCNTs is statistically significant in defining the properties of viscosity, conductivity, and surface tension, while the effect of temperature is statistically significant in all the studied parameters, presenting a p-value < 0.05. The interaction between concentration and temperature is statistically significant only in conductivity and surface tension.
The same conclusions cannot be taken when referring to water media. The effect of concentration and temperature on viscosity, density and conductivity are not statistically significant (p-value > 0.05), as proved by the experimental data, meaning that other experimental factors that were not considered in this study are affecting the physicochemical properties under evaluation. The only parameter in which both factors are statistically significant, including its interaction, is the surface tension, with a p-value < 0.0001.
For both dispersion media, the statistical analysis corroborates the experimental data obtained in this work.

Conclusions
The effect of MWCNTs and AgMWCNTs composites on the physicochemical properties of choline chloride-based DES, glyceline, and water was investigated, in the range of concentration from 0.2 to 1.0 mg cm −3 . The experimental results could be interpreted in terms of the hole theory, in which the ion transfer in DES is limited by the availability of holes of suitable size within the structure of the liquid, nevertheless, the effect of MWCNTs on viscosity and conductivity cannot be explained solely in terms of the hole theory and the contribution of the self-lubricating nature of carbonous materials has to be considered to explain the experimental results.
The measurements of viscosity, density, conductivity, surface tension and refractive index have been conducted. An increase in temperature was shown to result in a decrease in viscosity, density, and surface tension. The conductivity increases with an increase in temperature since it is controlled by ionic mobility.
The introduction of MWCNTs dispersions to glyceline and water led to an increase in conductivity, surface tension and refraction index. However, in glyceline, the presence of carbon nanomaterials led to a decrease in viscosity. The same study carried out in water led to an increase in viscosity, proving the exceptional properties of this choline chloride-based DES.
In general, the use of AgMWCNTs leads to a slight improvement of the dispersion properties, when compared to the use of MWCNTs dispersions. Conductivity is the physicochemical property that benefits the most from the use of AgMWCNTs, allowing to achieve up to 20 × enhancement of conductivity when MWCNTs dispersions are replaced by AgMWCNTs dispersions.
The dispersion of MWCNTs and AgMWCNTs is an exciting area of future research, in particular, their dispersion in eutectic mixtures. The obtained physicochemical properties of the MWCNTs/AgMWCNTs-glyceline dispersions will path the way to achieve exciting breakthroughs in carbon dispersions in the coming years, for several applications, especially in solid-state electrolytes and electrodes for energy storage devices.
Supplementary Materials: The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/nano12010099/s1, Table S1: parameters for the electrodeposition process of AgMWCNTs. Data from Brandão et al. [24] ** Figure S1: densities (ρ/g cm −3 ) of A-E mixtures (a-e) and comparison between samples at 0.4 mg cm −3 (f) in glyceline as a function of temperature; Table S2: densities of MWCNTs and AgMWCNTs mixtures in glyceline as a function of temperature for 0.4 mg cm −3 ; Table S3: densities of MWCNTs mixtures (0.2-1.0 mg cm −3 ) in glyceline as a function of temperature; Figure S2: densities (ρ/g cm −3 ) of A-E mixtures (a-e) and comparison between samples at 0.4 mg cm −3 (f) in water as a function of temperature; Table S4: densities of MWCNTs and AgMWCNTs mixtures in water as a function of temperature for 0.4 mg cm −3 ; Table S5: densities of MWCNTs mixtures (0.2-1.0 mg cm −3 ) in water as a function of temperature; Figure S3: plot of logarithm of the viscosity against the reciprocal value of the absolute temperature (1/T) for of A-E mixtures (a-e) and comparison between samples at 0.4 mg cm −3 (f) in glyceline; Table S6: viscosity of MWCNTs and AgMWCNTs mixtures in glyceline as a function of temperature for 0.4 mg cm −3 ; Table S7: viscosity of MWCNTs mixtures (0.2-1.0 mg cm −3 ) in glyceline as a function of temperature; Figure S4: plot of logarithm of the viscosity against the reciprocal value of the absolute temperature (1/T) for of A-E mixtures (a-e) and comparison between samples at 0.4 mg cm −3 (f) in water; Table S8: viscosity of MWCNTs and AgMWCNTs mixtures in water as a function of temperature for 0.4 mg cm −3 ; Table S9: viscosity of MWCNTs mixtures (0.2-1.0 mg cm −3 ) in water as a function of temperature; Table S10: the calculated values of the activation energies of viscous flow (E η ) and conductivity (Eτ) at different concentrations of MWCNTs and AgMWCNTs composites (A-E), in glyceline; Table S11: the calculated values of the activation energies of viscous flow (E η ) and conductivity (Eτ) at different concentrations of MWCNTs and AgMWCNTs composites (A-E), in water.; Table S12: viscosity-temperature model parameters for AgMWCNTs samples dispersed in glyceline; Table S13: viscosity-temperature model parameters for AgMWCNTs samples dispersed in water; Table S14: surface tension of MWCNTs mixtures (0.2-1.0 mg cm −3 ) in water as a function of temperature; Table S15: surface tension of MWCNTs and AgMWCNTs mixtures in water as a function of temperature for 0.4 mg cm −3 ; Table S16: surface tension of MWCNTs mixtures (0.2-1.0 mg cm −3 ) in glyceline as a function of temperature; Table S17: surface tension of MWCNTs and AgMWCNTs mixtures in glyceline as a function of temperature for 0.4 mg cm −3 ; Figure S5: surface tension of glyceline/water mixtures with MWCNTs and AgMWCNTs as a function of carbon concentration (a) and temperature (b and c); Table S18: average hole size (Å) according to the hole theory for glyceline at different concentrations of AgMWCNTs composites (A-E) at 20 • C; Table S19: average hole size (Å) according to the hole theory for glyceline at different temperatures (20-60 • C) of AgMWCNTs composites (A-E) at 0.4 mg cm −3 ; Table S20: conductivity of MWCNTs mixtures (0.2-1.0 mg cm −3 ) in water as a function of temperature; Table S21: conductivity of MWCNTs and AgMWCNTs mixtures in water as a function of temperature for 0.4 mg cm −3 ; Table S22: conductivity of MWCNTs mixtures (0.2-1.0 mg cm −3 ) in glyceline as a function of temperature; Table S23: conductivity of MWCNTs and AgMWCNTs mixtures in glyceline as a function of temperature for 0.4 mg cm −3 ; Figure S6: plot of logarithm of the conductivity against the reciprocal value of the absolute temperature (1/T) for of A-E mixtures (a-e) and comparison between samples at 0.4 mg cm −3 (f) in glyceline; Figure S7: plot of logarithm of the conductivity against the reciprocal value of the absolute temperature (1/T) for of A-E mixtures (a-e) and comparison between samples at 0.4 mg cm −3 (f) in water; Table S24: conductivity-temperature model parameters for AgMWCNTs samples dispersed in glyceline; Table S25: conductivity-temperature model parameters for AgMWCNTs samples dispersed in water; Table S26: refractive index of MWCNTs mixtures (0.2-1.0 mg cm −3 ) in water as a function of temperature; Table S27: refractive index of MWCNTs and AgMWCNTs mixtures in water as a function of concentration at 20 • C; Table S28: refractive index of MWCNTs mixtures (0.2-1.0 mg cm −3 ) in glyceline as a function of temperature; Table S29: refractive index of MWCNTs and AgMWCNTs mixtures in glyceline as a function of concentration at 20 • C; Figure S8: effect of concentration of MWCNTs and AgMWCNTs composites on refractive index in glyceline (a) and water (b) media, at 20 • C; Table S30: two-way ANOVA between the groups of MWCNTs and AgMWCNTs samples dispersed in glyceline with different temperatures (20-60 • C) and concentrations (0-1.0 mg cm −3 ); Table S31: two-way ANOVA between the groups of MWCNTs and AgMWCNTs samples dispersed in water with different temperatures (20-60 • C) and concentrations (0-1.0 mg cm −3 ).